1. Field of the Invention
The present invention relates to a transfer assembly of an electrophotographic printer, and more particularly to a transfer assembly capable of avoiding variation in transfer efficiency, thereby maintaining stable printing quality.
2. Description of the Prior Art
Hitherto, in an electrophotographic printer, a charged photoconductor drum is illuminated with a light source to form an electrostatic latent image on a surface thereof, developing is performed upon adhesion of toner to the electrostatic latent image, and then the thus obtained toner image is transferred to a recording medium.
A transfer assembly of a conventional electrophotographic printer will be explained hereinafter. FIG. 3 shows a schematic representation of the transfer assembly of the conventional electrophotographic printer, and FIG. 4 shows a view for explanation of a transfer operation. In the FIGS., a transfer unit 10, which is made of metal, is disposed against a photoconductor drum 7, and is grounded to the frame of a power supply unit, not shown.
A recording medium 2 fed from a recording medium cassette is transmitted by a pressure roller 5a and a feed roller 5b, and reaches the photoconductor drum 7. An image has been formed on the surface of the photoconductor drum 7 with a toner charged with negative electricity (-). When the recording medium 2 arrives at the photoconductor drum 7, the transfer unit 10 produces the positive electric charge (+) as shown in FIG. 4 and transfers the image on the surface of the photoconductor drum 7 to the surface of the recording medium 2. The recording medium 2 to which the toner image is transferred is heated when passing through a heating roller 12a and a backup roller 12b. so that the toner is fused on the surface of recording medium 2.
During a process in which the recording medium 2 picked up from the recording medium cassette is transmitted through the pressure roller 5a and the feed roller 5b. and reaches the transfer unit 10. the recording medium 2 is charged with positive electricity (+). This positive electric charge (+) is utilized together with the positive electric charge (+) produced by the transfer unit 10 to transfer the image on the surface of the photoconductor drum 7 to the surface of the recording medium 2.
The electric charge stored in the recording medium 2 flows. when passing through the transfer unit 10. through a metallic frame of the transfer unit 10 into the power supply unit frame ground. and is grounded via a frame ground wire.
In the transfer assembly of the conventional electrophotographic printer as stated above. however. when the recording medium 2 touches the metallic frame of the transfer unit 10 until it reaches through between the pressure roller 5a and the feed roller 5b. the positive electric charge (+) on the recording medium 2 flows at once through the metallic frame of the transfer unit 10 to the power supply unit frame ground. and then is grounded via a frame ground line. Hence, variations of in transfer efficiency of the transfer assembly 10. are produced thereby decreasing a printing quality.
On the other hand, in order to prevent such a phenomenon, if there is provided an isolation member at a portion in the transfer unit 10, which portion the recording medium 2 touches so that electric charge is grounded, there is a disadvantage in that the positive electric charge is accumulated on the isolation member and invites adhesion of toner, thereby staining a back surface of the recording medium 2.